I tried to send a half signed multisig transaction to the testnet but it got rejected.
I think this is by design but has anyone some hard facts on this?
EDIT: I think https://en.bitcoin.it/wiki/BIP_0010 was the case I was looking for.
The network will only forward transactions that are valid. That is they only claim so far unclaimed outputs and contain all the necessary signature scripts. Your transaction does not contain valid signature scripts, hence it's not valid and will be dropped by the nodes in the network.